1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus.
2. Description of the Related Art
Intermediate transfer type color image forming apparatuses are widespread in use. In such image forming apparatuses, a toner image each formed on a photosensitive element is primarily transferred onto an intermediate transfer member, and a color image on the intermediate transfer member is secondarily transferred onto a recording medium (sheet). The image forming apparatuses can be used for various types of sheet media such as thin sheet, thick sheet, postcard, and envelope, and therefore has an advantage of having high versatility. As the intermediate transfer member, an intermediate transfer drum or an intermediate transfer belt is generally used.
However, when a sheet with a certain level of thickness enters into a secondary transfer unit, the speed of the intermediate transfer member being driven at a certain speed drops for a short period of time. This disturbs image forming operation in the primary transfer unit.
Furthermore, along with downsizing of color image forming apparatuses, the secondary transfer unit and a fuser become adjacent to each other, and transfer and fixation of the image can be performed simultaneously on the sheet (when fixation is performed at a front end of one sheet, the image is transferred to a rear end of the sheet). At this time, when a sheet with a certain level of thickness enters into the fuser, the speed of a fuser roller or a fuser belt being driven at a certain speed drops for a short period of time. This disturbs image forming operation in the secondary transfer unit as in the intermediate transfer unit.
There is an image forming apparatus adopting a simultaneous transfer and fixing method in which transfer and fixation of the toner image onto the sheet is performed simultaneously (at a time). In this case also, when a sheet with a certain level of thickness enters into a transfer-fixing unit, the speed of the intermediate transfer member being driven at a certain speed drops for a short period of time, thereby causing a problem that the image is disturbed in the primary and secondary transfer units, as at the time of entering into the secondary transfer unit.
Japanese Patent Application Laid-open No. 2005-107118 discloses a conventional color image forming apparatus, in which the speed of a belt is made constant by changing a speed control amount with respect to a driving source of an endless belt at a preset predetermined timing, by a predetermined amount, and for predetermined duration.
In the conventional image forming apparatus, however, a unit that detects a mechanical property of a sheet such as a thickness sensor is required.
Moreover, because a control target value preset based on the type, thickness and width of a sheet is used, it is difficult to perform optimum control with respect to all usable sheets. Furthermore, even with the same sheet, the thickness and firmness change according to environmental conditions such as temperature and humidity, and fluctuation of speed caused thereby is different, and therefore, optimum control is difficult to perform.
Besides, it is required to store control target values corresponding to various types of sheets. With an increase in the type of sheets that can be handled, a storage unit is required to have a larger memory capacity.